This invention relates generally to soldering alloys, and more specifically relates to alloy compositions useful in machine soldering applications.
The use of machine (or "automated") soldering operations, is an indispensable aspect of many modern industrial applications. A particularly noteworthy example is the use of such techniques in the electronics and related industries. In a typical operation common in such industries, for example, an integrated circuit package may be soldered to the conductive pattern of a printed circuit board, by disposing the package on the board with the leads from the package inserted into and through openings in the board. These board openings may, for example, be placed through, and interconnected by the aforementioned printed circuit pattern. Electrical connection is thereupon effected between the leads and printed pattern, by contacting the lower surface of the board with a volume of molten solder in a suitable reservoir.
As is known in the art of machine soldering, various techniques may be used for effecting contact between the workpiece to be soldered and the molten volume of solder in its reservoir. In some instances, for example, so-called dip soldering techniques are used. These are basically static in nature, which is to say that the workpiece is simply immersed into the reservoir of molten soldering alloy and withdrawn after a specified period. In other instances dynamic techniques are utilized. Thus the workpiece -- e.g. a pre-fluxed circuit board -- may be passed face downward along the surface of the solder bath, or it may be conveyed across the crest of a standing wave or weir of flowing solder pumped out of a nozzle communicating with the molten reservoir.
As opposed to the relatively simple requirements for alloys used in hand-soldering, certain rather stringent requirements are imposed upon the soldering alloys used in the aforementioned automated soldering environments. The soldering alloys so utilized, must, among other things, display rheological characteristics such as to provide high mobility and free-flow of the melt. The flow and wetting characteristics of the melt must assure that effective and rapid wetting of the workpiece occurs even under the stringent requirements of the dynamic mode of operation mentioned above.
Particularly where soldering of circuit boards or other workpieces including solid state electronic devices is involved, the time and temperature of exposure during soldering are highly significant. This is true in that many of the electronic devices thus exposed, are quite sensitive to heat damage. In general, therefore, the alloy used should have as low a melting point as possible. The rheological characteristics and time and temperature of exposure are also significant in that it is desirable to limit diffusion into the materials of the workpiece. Typically it is desirable to work at about 120.degree.F above the melting point in the type of environment of interest.
A particularly significant aspect of automated soldering from a molten reservoir, is the problem of avoiding production of cold or disturbed joints in the soldered components. Again this problem can become particularly acute where a dynamic mode of machine soldering is utilized, such as, for example, the wave soldering techniques previously alluded-to.
In order to minimize the aforementioned difficulties encountered in automated soldering, it has been generally recognized desirable, to utilize a soldering alloy which is close to the tin-lead eutectic composition. Cost factors aside, the ideal composition for such purposes is indeed the tin-lead eutectic, i.e. a nominally 63% tin, 37% lead alloy, which, as is well-known, has a sharp and distinct melting point at 361.degree.F. Use of such eutectic composition results in fine-grained joints having excellent mechanical properties.
In practice, the eutectic composition above mentioned, may be modified by addition of small fractional percentages of certain elements. For example, Federal Specification QQ-S-571 calls for 0.10 to 0.25% antimony -- which element inhibits the risk of white-to-grey tin transformation in soldered joints exposed to temperatures less than 55.8.degree.F. or 13.2.degree.C. The said specification also permits the presence of certain other impurities within prescribed limits.
Notwithstanding the advantages of the aforementioned eutectic tin-lead composition, it is in general not widely utilized in automated soldering processes, because of the very cost of tin. This factor is so compelling, that it has been deemed desirable to utilize tin/lead alloys wherein the content of tin is lowered to such degree as does not seriously impair the usefulness of the alloy in machine soldering applications. In general, it has been considered in the past that the practical limit of such displacement extended no further than about the use of a so-called "60/40" solder, i.e. a solder with a nominal composition of 60% tin and 40% lead. Theoretically a 60/40 solder has an approximate solidus-to-liquidus range of 13.degree.F. It may, however, be noted in this connection that as used in this specification, the term "60/40" solder refers to the nominal composition identified as "Sn 60" in the aforementioned Federal Specification QQ-S-571. Taking this factor into consideration, the 60/40 alloy commercially available can, pursuant to the said Federal Specification, actually display a solidus-to-liquidus range as much as 17.degree.F, at the allowable 59.5% tin, and it is this latter numerical value which is intended hereinbelow, where reference is made to the "solidus-to-liquidus range of 60/40 solder".